1. Field of the Invention
The present invention relates to a telecommunication antenna intended to be placed onboard a telecommunication satellite, a payload of a telecommunication satellite comprising the antenna, and a telecommunication system using the payload and therefore the telecommunication antenna.
2. Description of the Related Art
In general and to date in the case of spatial telecommunication using geostationary satellites for the transmission of Ka-band multimedia services, one seeks to broaden the coverage provided by the telecommunication antenna(e) onboard the satellite and to increase the transmission capacity while ensuring a high C/I performance (payload to interfering signal ratio).
To obtain the expected system level performance, it is necessary to have telecommunications antennae that ensure sufficient spatial insulation between beams or their footprints, hereafter called elementary areas or spots, so as to allow reuses programmed in a fixed or dynamic manner of all or part of the frequency resources allocated to the system (reuse of frequencies).
Given the large number of spots to be produced, a large number of directional antennae must be installed on a same satellite platform, and it is also necessary to have large focal structures to achieve high isolation performance between beams associated with a severe pointing stability.
In our time, Ka-band multimedia programs use multiple-reflector antenna solutions. In fact, using several reflectors makes it possible to use large enough feeds to optimally illuminate the reflectors and thereby form fine beams with a high maximal directivity (high antenna efficiency).
The most recent satellite in Europe using this type of antenna is the operator Eutelsat's Ka-sat satellite. It provides European coverage using about 80 beams with a 0.45° angular opening generated by four reflectors measuring 2.6 meters in diameter. Each of these reflectors operates on a forward transmission downlink and on a return reception uplink. This communication system is provided to supply a total capacity of about 70 Gbits/s, the minimum I/C ratio on the coverage being around 14 dB.
It should be noted that the Ka-sat satellite could have used a single reflector measuring 2.6 meters in diameter. In this case, it would have been necessary to produce smaller illumination sources, which would have deteriorated the antenna's efficiency, in particular by increasing energy losses by spillover, typically from 4 to 6 dB. Since the C/I performance remains in the vicinity of 12 dB, the efficiency loss of the antenna would have caused a deterioration of the Effective Isotropic Radiated Power (EIRP), which would amount to a notable and unwanted loss of capacity of the telecommunications system.
Today, several missions are distinguished, from the coverage of a large region, e.g. Europe, to coverage for one or a small number of several European countries.
The study of coverage concerning one to three countries is currently the subject of considerable research and development. For example, the provision of a high-capacity multimedia telecommunication satellite having a coverage area for a country the size of France is contemplated.
In these systems, which are being studied and developed, capacity increases are still being sought through the use of a broader allocated bandwidth when allowed by the regulations, or through the reuse of the spectrum in reduced areas using very fine narrow beams.
Overcoming this gap in terms of capacity then requires the use of more fine beams.
However, the contribution capacities of the current platforms and launch vehicles do not make it possible to consider solutions with multiple reflectors having a diameter greater than 3 or 3.5 meters.
Thus, the use of four reflectors measuring 3.2 meters in diameter currently corresponds to a contribution limit configuration on a satellite to enter a fairing of a launch vehicle.
For this configuration of antennas with four reflectors and by best optimizing the high weight determining parameters of the system, a capacity is obtained of about 65 Gbits/s with 36 beams over France.
In this limit configuration, the feeds are optimized for the four reflectors and the spillover losses are about 2 dB for a minimal C/I in the vicinity of 9 dB.
Moreover, when the number of beams increases, the observed C/I becomes very low and, despite the use of frequencies, the capacity decreases.
The technical problem is to increase the transmission capacity of the satellite under operating conditions of the satellite identical to those presented for the limit configuration in terms of power consumed by the multimedia payload of the satellite, frequency band allocated to the downlink, characteristics of the terminals and mating limitation within a satellite intended to enter a fairing of a launch vehicle.